TD-DFT studies on the electronic structure of imidazole bound vanadate in vanadium containing haloperoxidases (VHPO)

Time dependent density functional theory (TD-DFT) is utilized to study the electronic structure of the active site of vanadium containing haloperoxidases (VHPOs). Calculations on the species, modeled for pentacoordination of the vanadium atom as found in the active site of the enzyme are performed for different geometries and protonation states of the vanadate moiety. Calculations are also carried out on structures in which vanadate is surrounded by all of the seven amino acid residues which constitute the active site of the enzyme. Two such structures with mono and doubly protonated vanadate were considered. Excitations calculated by TD-DFT, using statistical average of the orbital model exchange-correlation potential (SAOP), reveal that the excitation behavior of doubly protonated vanadate species located in the protein model matrix closely resembles the experimentally observed optical transition for the holoenzyme. Implications of the hydrogen bonding interactions through the protein environment as well as the variation of the protonation state of the active site vanadate are discussed. For the vanadate a rather large distortion from the idealized trigonal bipyramidal arrangement is observed.